Users can create requests through the customer portal without creating an account. They receive a confirmation email with a "View Request" link, however when clicking the link they are prompted to create an account or login.
In Jira Settings > Jira Service Management > Customer Access under Portal Access "Allow customers to create accounts is bubbled in and "Customers can access and send requests from the portal without logging in" is CHECKED.
IMO this means they should be able to access their requests with just the link in the email and not needing to login.